Magnetic systems for pick-ups working according to the moving coil principle are usually constructed as a ring-shaped yoke circuit consisting of a permanent magnet, pole shoes and an air gap in which the armature or coil system of the pick-up is placed.
The air gap may be positioned at various locations in the magnetic circuit and thus utilize the magnet more or less efficiently, corresponding to greater or smaller flux density in the air gap with the same magnetic volume.
Usually, the magnet is the most expensive component in the magnet circuit, and until a few years ago mainly Alnico magnet types were used. These magnets are characterized by an energy product (BH).sub.max of 4 to 10M gauss- sted for the anisotropic types corresponding to a magnetic induction B.sub.r of about 7 to 13K gauss and coercive forces H from 0.7 to 1.9K rsted. It is moreover characteristic of these magnets that with a square cross-section the length-width ratio will be&gt;1 corresponding to bar shape owing to the relatively low coercive force. When these magnets are loaded with a yoke circuit, the magnetic induction B.sub.r declines along a working line depending upon load and magnet type. To reduce the costs of magnet volume, the circuit is usually dimensioned to the working point which gives (BH).sub.max.
Owing to the elongate shape there will be a considerable loss, also in an incorporated state, because of stray fields around these magnets. Where possible, the magnet is therefore positioned preferably directly up to the active air gap.
Recent years have seen the development of high energy magnets based on samarium cobalt alloys with energy products of 20 to 27M gauss- rsted.
In particular the very high coercive force of 5 to 8K sted makes them interesting for use in pick-ups. One reason is that the magnets give approximately the same B.sub.r with a smaller volume, another is that the high coercive force entails that optimum dimensioning gives disc-shaped magnets instead of bar-shaped ones, which is very expedient because of the miniaturization taking place within the pick-up field.
In moving coil pick-up systems, it is essential to have a powerful homogenous field in the air gap where the windings move.
The high energy magnets have the field radiation concentrated on the pole faces to a higher degree than an isotropic Alnico type, where a major part of the radiation occurs from the sides at right angles to these.
To utilize the properties of the high energy magnets optimally, at least one of the pole faces of the magnet should restrict the air gap. The air gap induction is increased significantly by arranging a U-shaped yoke, illustrated in FIG. 2, in the usual manner, said yoke connecting the two pole faces with each other around the air gap. However, considerations of space in moving coil phone cartridges and also process-technical considerations in the production may cause another structure of the magnet circuit, which is the idea of the invention.